SF bug [#466173] unpack TypeError unclear

Replaced 3 instances of "iter() of non-sequence" with
"iteration over non-sequence".
Restored "unpack non-sequence" for stuff like "a, b = 1".

Objects/abstract.c

@@ -1859,7 +1859,7 @@ PyObject_GetIter(PyObject *o)
 	if (f == NULL) {
 		if (PySequence_Check(o))
 			return PySeqIter_New(o);
-		PyErr_SetString(PyExc_TypeError, "iter() of non-sequence");
+		PyErr_SetString(PyExc_TypeError, "iteration over non-sequence");
 		return NULL;
 	}
 	else {

Objects/classobject.c

@@ -1839,7 +1839,7 @@ instance_getiter(PyInstanceObject *self)
 	}
 	PyErr_Clear();
 	if ((func = instance_getattr(self, getitemstr)) == NULL) {
-		PyErr_SetString(PyExc_TypeError, "iter() of non-sequence");
+		PyErr_SetString(PyExc_TypeError, "iteration over non-sequence");
 		return NULL;
 	}
 	Py_DECREF(func);

Objects/typeobject.c

@@ -3261,7 +3261,7 @@ slot_tp_iter(PyObject *self)
 	PyErr_Clear();
 	func = lookup_method(self, "__getitem__", &getitem_str);
 	if (func == NULL) {
-		PyErr_SetString(PyExc_TypeError, "iter() of non-sequence");
+		PyErr_SetString(PyExc_TypeError, "iteration over non-sequence");
 		return NULL;
 	}
 	Py_DECREF(func);

Python/ceval.c

@@ -1556,8 +1556,12 @@ eval_frame(PyFrameObject *f)
 			else if (unpack_iterable(v, oparg,
 						 stack_pointer + oparg))
 				stack_pointer += oparg;
-			else
+			else {
+				if (PyErr_ExceptionMatches(PyExc_TypeError))
+					PyErr_SetString(PyExc_TypeError,
+						"unpack non-sequence");
 				why = WHY_EXCEPTION;
+			}
 			Py_DECREF(v);
 			break;